This R21 application is in response to NHLBI RFA-HL-12-004 Maximizing the Scientific Value of the NHLBI Biologic Specimen Repository: Scientific Opportunities (R21). Acute Lung Injury and the Acute Respiratory Distress Syndrome (ALI/ARDS) remain common causes of morbidity and mortality in the intensive care unit. Novel therapies for prevention and treatment are needed. Pulmonary vascular dysfunction leading to pulmonary arterial hypertension may be an important contributing factor to outcomes from ALI/ARDS. However, the pathophysiology of pulmonary vascular dysfunction in ALI/ARDS is poorly understood. One potential etiologic factor is circulating cell-free hemoglobin, a potent vasoconstrictor. Elevated levels of circulating free hemoglobin have been found to be the primary cause of pulmonary arterial hypertension in sickle cell anemia and have been linked to poor clinical outcomes in other patient populations including our own preliminary studies of patients with sepsis, trauma and ALI/ARDS. Hemoglobin-induced pulmonary vascular injury and pulmonary hypertension in ALI/ARDS may be driven by the potent pro-oxidant effects of hemoglobin. The proposed studies will test the hypothesis that: High circulating levels of free hemoglobin in patients with acute lung injury contribute to pulmonary vascular dysfunction, non-pulmonary organ failure and poor clinical outcomes. We aim to determine whether free hemoglobin is an important determinant of pulmonary vascular dysfunction in clinical ALI/ARDS (Aim 1) by testing the association between plasma free hemoglobin levels and pulmonary vascular dysfunction (as measured by pulmonary artery pressure, pulmonary vascular resistance, transpulmonary resistance), cardiac dysfunction (as measured by cardiac output and mixed venous oxygen saturation), non-pulmonary organ dysfunction, and biomarkers of endothelial injury and mortality in patients with ALI/ARDS enrolled in a large clinical trial. We wll determine why patients with ALI/ARDS have elevated levels of free hemoglobin (Aim 2) by measuring circulating levels of the two major hemoglobin binding proteins (hemopexin, haptoglobin) to determine if low levels of these proteins are the primary mechanism leading to elevated levels of circulating free hemoglobin in patients with ALI/ARDS. Finally will determine how elevated free hemoglobin contributes to poor outcomes in this patient population (Aim 3) by measuring markers of lipid peroxidation (isoprostanes and isofurans) to determine if oxidant stress underlies the damaging effects of free hemoglobin. In summary, this innovative study will identify a novel mechanism of pulmonary vascular dysfunction in ALI/ARDS, will determine why some patients have elevated levels of free hemoglobin and whether free hemoglobin causes lipid peroxidation. This study will be conducted by an experienced investigator with a strong support team and internationally recognized consultants. Using sound experimental and statistical methods and a well characterized patient cohort, the proposed studies are likely to generate significant findings that will advance our understanding of ALI/ARDS.